Bacteriophage Mu possesses an unusual recombination system which recognizes a specific attachment site on the phage chromosome and recombines it with an apparently random sequence in the host DNA. The long-range objective of this laboratory is to understand, at the molecular level, the mechanism by which this recombination occurs. This objective will be pursued in three ways: 1) By constructing a DNA substrate suitable for the characterization of the Mu integration process and using that substrate to analyze the various stages of integration in vivo and in vitro and to isolate and characterize mutations in the sites and functions necessary for the integration process. 2) By isolating and characterizing host mutants unable to allow Mu integration and development and mutants of Mu able to overcome the host defects. 3) By continued physical and genetic analysis of host deletion mutations induced by Mu.